(1) Field of the Invention
The present invention relates to a system and method for designing a circuit board including a printed-circuit board or a multi-chip module (MCM), with the use of a CAD (computer aided design) system.
(2) Description of the Related Art
FIG. 1 is a flowchart depicting a conventional process of designing a printed-circuit board with the use of a CAD system.
First of all, the designer designs an electric circuit showing the electric connection among all the components to be mounted on the printed circuit board, with the use of CAD for circuits (step 2101). To be more specific, the designer designs a fundamental electric circuit (step 2102), and adds noise reduction components which are necessary for preventing reflection, delay, and noise (step 2103).
Then, a net list having information on the electric connection is inputted to CAD for printed circuit boards, to interactively set a layout. The layout includes the placement of components and the connection of routing pattern (step 2104).
The designer then examines whether the printed-circuit board (hereinafter PC board) properly operates as is expected. The examination may be done through a simulation or by actually producing a PC board and applying a probe (steps 2105 and 2106).
As a result, it may be found that the PC board does not work properly or cannot put out expected performance due to electric characteristics such as signal delay, noise, or electromagnetic radiation. In such a case, the designer modifies the rated value, such as the resistance value or capacity value of each noise reduction component (step 2108), and examines through a simulation whether the PC board operates as is expected (steps 2105, 2106, and 2107).
In the case that the PC board still does not work properly or cannot put out expected performance, the layout including the placement of components and routing foils or route length is modified (step 2104), and it is examined whether the board works properly as is expected (steps 2105, 2106, 2107, and 2109).
If the PC board still does not work properly or cannot put out expected performance, additional noise reduction components such as a resistance or capacitor are provided (step 2103), and the PC board is examined through a simulation or another method whether it works properly as is expected (steps 2104, 2105, and 2106).
Through these steps, the designer applies the PC board with appropriate counterparts to make the electric performance sufficient. The steps 2105 to 2108, steps 2104 to 2109, and steps 2103 to 2106 may not be necessarily proceeded in these orders because they are carried out interactively. It is also possible to go back to step 2102, if needed, to design the fundamental electric circuit.
Thus, realizing that the PC board operates properly, the designer forwards the obtained PC board design information to the production process.
Some CAD systems are provided with a function of automatically setting a layout (National Technical Report Vol. 39 No. 2 Apr. 1993, "High-Density Printed Circuit Board Designing System Based on Artificial Intelligence Techniques", pp. 84-89, published by Matsushita Electric Industry). However, the high-density printed circuit board designing system does not take it into account to add noise reduction components to the circuit or modify the layout and the rated values of the noise reduction components, in accordance with the electric characteristics of the PC board which has been automatically set a layout. Consequently, the designer needs to perform the examination of the electric characteristics of the PC board and corresponding correcting process interactively.
U.S. Pat. No. 5,187,668 discloses a placement optimization system aided by CAD. This system does not take the electric characteristics of the component-added circuit into account.
U.S. Pat. No. 5,046,017 discloses a routing design for semiconductor integrated circuit, which discloses only the setting of a routing layout in accordance with the electric characteristics, and does not take the modification of the circuit in accordance with the electric characteristics.
Thus, according to the conventional PC boards, it is necessary for the designer to interactively continue the modification of the layout or the rated values of the components, or the addition of noise reduction components, by means of a simulation or another method. These procedures for adding noise reduction components to the circuit requires many man-hours until it has been finally checked that the board operates properly.
The following is a description on the usage of a conventional design rule.
Generally, design rule means restrictions to observe in designing a PC board, in order to properly produce and operate the designed PC board. The following are examples: "close parallel routes are vulnerable to cross talk noise, so that the maximum length of these parallel routes is made 10 centimeters", and "a component, which is inserted with the use of an insert machine, should have 1 centimeter margin on its sides, so as to be held by the machine".
Although these design rules may be slightly different depending on the type of PC boards to be designed or their production methods, if PC boards have different design rules from each other, it not only takes time to form the different design rules, but the designer must design every one of design rule. In view of these inconveniences, design rules are conventionally predetermined depending on the types of PC boards.
Thus, these design rules are predetermined like a greatest common divisor. In other words, PC boards can be produced without any serious problem as far as they observe the design rules, and as a result, some boards are produced with too lenient design rule. For example, a PC board which could have been designed and produced into four layers actually had six layers because it observed the design rule, and another PC board was produced in a larger size than expected. These problems lead to a high production cost, and prevents an apparatus provided with such a PC board from being minimized.
On the other hand, even a lenient design rule does not necessarily meet the requirements of every PC board. There are cases that a board which satisfies the design rule does not operate properly when it is actually produced.